Prior art techniques used in determining the range between two spacecraft vehicles for automatic rendezvous and docking of such vehicles, includes vehicle radar, man in loop estimates, global positioning systems, lasers, loran, and video guidance sensor systems for processing optical images in determining range. The video guidance sensor system approach, which is of particular importance here, is based on the concept of using captured and processed images to determine the relative positions and attitudes of a video guidance sensor and a target. However, conventional video guidance sensor systems tend to be bulky, heavy, and slow, and have high power requirements or demands.
One prior video guidance sensor system uses two lights of predetermined wavelengths to illuminate a target. The target includes a pattern of filtered retroreflectors to reflect light. The filtered retroreflectors pass one wavelength of light and absorb the other. Two successive pictures or images are taken of the reflected light and the two images are then subtracted one from the other, thereby allowing for target spots to be easily tracked. However, due to its size, weight, power requirements and speed, the prior art video guidance sensor system is of limited use in applications requiring fast tracking of moving objects. Such a system is described, for example, in R. Howard, T. Bryan, M. Book, and J. Jackson, “Active Sensor System for Automatic Rendezvous and Docking,” SPIE Aerosense Conference, 1997, which is hereby incorporated by reference.
Another prior art video guidance sensor system uses a CMOS imaging chip and a digital signal processor (DSP) in order to provide higher-speed target tracking and higher-speed image processing. The faster tracking rates result in a more robust and flexible video guidance sensor. Because of these faster tracking rates, the video guidance sensor system can track faster moving objects or provide more data about slower moving objects. This video guidance sensor system is designed to be less complex, consume less power and volume and weigh less than previous systems. However, this video guidance sensor system has limitations with respect to extended rangefinding. Such a system is described, for example, in R. Howard, M. Book and T. Bryan, “Video-based sensor for tracking 3-dimensional targets,” Atmospheric Propagation, Adaptive Systems, & Laser Radar Technology for Remote Sensing, SPIE Volume 4167, Europto Conference, September 2000, and in R. Howard, T. Bryan, and M. Book, “The Video Guidance Sensor: Space, Air, Ground and Sea,” GN&C Conference, 2000, which are also hereby incorporated by reference.
A general problem with the conventional video guidance systems of particular concern here is that beyond the distance at which the spots of the target on the vehicle merge into a single “blob,” the system cannot determine from the image the range to the target and can only determine the bearing to the target. This limitation with respect to rangefinding creates a gap between the maximum tracking range for the vehicle guidance sensor system and the minimum safe range, when using simple absolute Global Positioning System (GPS) state vector navigation. In this regard, a two to five kilometer range capability has been discussed as the safe range for the transition from simple GPS.
A number of approaches have been proposed to “fill the gap” in question, but these involve complex sensor arrangements which add further sensors, more mass, and greater power demands, frontal area and costs to the chase vehicle incorporating these arrangements. Many modern laser rangefinders use simple pulse detectors which require pulse laser illuminators with high voltage power supplies (i.e., voltage greater than 28 VDC), capacitive discharge pulse laser drivers, special pulse laser diodes, and high speed detectors, and these requirements are problematic for space applications. Other conventional rangefinders use sinusoidal amplitude modulation and phase measurement which requires complex circuitry and precise gain control and/or complex frequency control circuitry for range measurements.